Portland cement is prepared conventionally by grinding limestone and clay and/or other materials to a fine powder, mixing thoroughly, and burning the mixture in a long rotary kiln. At the hot zone of the kiln, where the temperature approaches 3000.degree. F., the mixture is sintered and fused together into nodules called clinker which is ground to a find powder. Substantial fuel is required to serve the energy demand for this low cost, high volume conventional cement material, and, as in all energy consuming industries, fuel shortages curtail cement production. Hence, a need exists for low energy consuming cements in building materials.
The possibility of fossil fuel shortages and accompanying demand for high energy fuels has spurred efforts to recover geothermal energy as well as petroleum gas deposits trapped in very deep cavities and to liberate these reserves. Nuclear devices have been proposed as fracturing methods to stimulate production of high yields. However, the combination of inherently high subterranean temperature and the added thermal influx from nuclear stimulation poses a serious threat to the stability of cements used to reinforce the casing and walls of the wells and to secure the casings to wells in desired configurations for attachment to aboveground piping and valve assemblies. The high temperature gradients that define the thermal profile of these walls constitute a source of variable stress between cement and steel casing. The high coefficient of thermal expansion of metal compared to concrete (in the range of 5-50 to 1) can induce adhesive failure if the cement bond is not satisfactory. Conventional Portland cements rapidly lose adhesive strength at elevated temperatures (i.e., 300.degree. F. and higher) and possess only marginal strength under optimum temperature conditions. For example, the shear bond of Portland cement to Re Bar (conventional steel reinforcing rods) averages 500 psi while the shear bond of Portland cements to conventional polished steel is only 50 psi. The added stress of thermal degradation would render a Portland cement useless in regard to adhesion in wells of the nature described above. Consequently, the need exists for a high strength cement exhibiting high shear bond and adhesion properties for the use described.
In the drilling of conventional oil and gas wells, casing is cemented not only to secure the casing in the bore hole but also to prevent communication between water, oil and gas-producing zones within the well. The typical procedure for cementing wells of this nature involves mixing Portland cement compositions at the well site and pumping the cement downwardly through casing and then outwardly and upward through a well between the casing and the wall of the bore hole. Ordinarily, a plurality of high pressure pumps and piping systems are required to guard against pump failure. In the event of pump failure, the cement sets almost immediately and hardens, thereafter requiring expensive drilling operations to salvage the well. Thus, it would be advantageous to provide a method of cementing a string of pipe in a bore hole whereby the cement will not set, harden or cure except by passage of time or by exposure of the cement to given temperature conditions that exist within the well or both. The present invention is directed to such method which eliminates the necessity of equipping cementing systems with duplicate pumps and related apparatus and which virtually eliminates the hazard of the cement setting before the desired time which might lead to loss of the well.
Applicant is aware of prior art in the field including U.S. Pat. Nos. 1,318,076, 1,852,672, 2,042,011, 2,238,930, 2,279,262, 2,302,913, 2,502,418, 2,586,814, 2,665,996, 2,682,092, 2,701,209, 2,805,719, 2,883,723, 2,895,838, 3,146,828, 3,180,748, 3,208,523, 3,244,230, 3,253,664, 3,317,643, 3,326,269, 3,736,163, 3,374,834, 3,435,899 and 3,581,825, and an article entitled "Effect of Jet Perforating on Bond Strength of Cement" by W. K. Godfrey, Journal of Petroleum Technology, pages 1301-14, November 1968. Of the foregoing, Applicant deems the most pertinent to be U.S. Pat. Nos. 1,852,672, 2,665,996, 3,146,828, 3,326,269 and 3,736,163.
The U.S. Pat. No. 1,852,672 patent is directed to pozzolanic activity of active clay minerals with alkaline earth oxides such as calcium and magnesium. The U.S. Pat. No. 3,326,269 patent teaches the fixation of free silica by the high temperature firing of mixtures of colloidal silica and the hydroxide of various colloidal polyvalent metals. Neither patent is concerned with reactions or products subject of the present invention.
The U.S. Pat. No. 3,146,828 teaches the use of heat to generate a rigidized mineral mass of highly porous silicate and silica of moderately low strength (1546 psi in the highest case). It teaches the use of the material to form a consolidating barrier of a permeable structure within the formation surrounding the well bore, the purpose of which is to aid the fracturing of formations and assure a continuous recovery of fluids. The bonding or cementitious material is defined as being sodiun silicate to which is added a zinc oxide stabilizing agent in the amount of 0.5 to 2.0 parts by weight to reduce the water sensitivity. The patent disclaims any benefits of greater amounts of zinc oxide (column 6, lines 54 to 59) and describes a process wherein sodium silicate solutions are dehydrated thermally at such a rate (indicated by the minimum temperature shown as 175.degree. F.) that a porous film results which serves as a binder. It has been found in practice that the cement of the U.S. Pat. No. 3,146,828 is a water-vapor sensitive product, one which is stable at low humidity but softens and degrades at 175.degree. at 100% relative humidity within 24 hours of exposure. When formulations such as those described in column 5, lines 6 through 75, and column 6, lines 1 through 60, are prepared and cured under closed autoclave conditions at 175.degree. F. and 100% relative humidity for 5 days, a plastic non-rigid product results unless the sample is allowed to dehydrate. Where evaporation is permitted or induced, the cement does harden. If such a formulation is charged into a well whose formation permits water vapor transfer from the cement, a hardening will result. If the formation is tight or if the cement is cast between non-porous steel casings as is often done, the cement may fail to set firmly and may not achieve the porosity claimed by the inventor as critical to the invention (column 5, lines 6-16). Thus the U.S. Pat. No. 3,146,828 formulations have definite limitations.
The U.S. Pat. No. 2,665,996 describes a hydrated calcium silicate-crystalline product that is unstable at temperatures above 450.degree. F. and reacts at room temperature as well as elevated temperatures. Furthermore, the U.S. Pat. No. 2,665,996 product does not have the property of adhesion to steel. By comparison, the composition of the present invention is a hydrothermally reacted product, inert or unreactive at room temperature while reactive at higher temperatures with the consequent benefits of remaining mobile or pumpable for long periods of time at less than the threshold of activation temperatures. Furthermore, the product of the present invention cures to a heat-resistant solid with great adhesive strength to steel and resistance to acids and is an amorphous, polymeric, non-crystalline and non-hydrous solid as compared with that of the U.S. Pat. No. 2,665,996.
U.S. Pat. No. 3,736,163 discloses a lightweight insulative product consisting of mineral wool type fibers bonded together with calcium silicate moieties which become partially mineralized during use. The product of the U.S. Pat. No. 3,736,163 is cured in any autoclave at 200 psi steam (400.degree. F.) for 3 hours, then dried and has negligible adhesion to metal and is a crystalline hydrate of dicalcium silicates that are initially partially dehydrated before released for use. The product of the U.S. Pat. No. 3,736,163 process survives temperatures of not greater than 1200.degree. F. because of stress relief by way of microfractures and microstrain dissipated by the fiber fillers. When the same compositions (875 pounds lime and 750 pounds of uncalcined diatomaceous earth, 150 pounds of anhydrous sodium silicate, 7 pounds of sugar, 60 pounds of nodulated mineral wool, 60 pounds of sulfite pulp fibers and 50 pounds of clay in 720 gallons of water) is autoclaved as described in the U.S. Pat. No. 3,736,163, a product which is a mixture of crystalline calcium silicates is obtained as confirmed by x-ray analysis.
By comparison, the vastly different composition of the present invention contains, for example, minimal amounts of water as compared with the U.S. Pat. No. 3,736,163. Minimal water in the product of the present invention allows reaction to form polymers of non-crystalline and non-hydrous solids that provide resistance to high temperature thereby dispensing with the necessity of fibers as in the U.S. Pat. No. 3,736,163 for the purpose of dissipating stresses.